Conventionally, in an image forming apparatus that employs an electrophotographic printing method, a charging device that employs a corona discharge system has been often used in, for example, a charging device for charging a photoreceptor, a transfer device for electrostatically transferring, to recording paper or the like, a toner image which is formed on the photoreceptor or the like, and a separation device for separating the recording paper or the like which electrostatically comes into contact with the photoreceptor or the like.
Such a charging device employing the corona discharge system generally includes a shield case having an opening section that faces a charge receiving material such as the photoreceptor and the recording paper, and a discharge electrode of a line or saw-tooth shape which discharge electrode is provided in a tensioned state in the shield case. Examples of this charging device include (a) a corotron that (i) applies a high voltage to the discharge electrode so as to generate corona discharge and, thereby, (ii) uniformly charges a charge receiving material, and (b) a scorotron that (i) applies a desired voltage to a grid electrode provided between a discharge electrode and a charge receiving material and, thereby, (ii) uniformly charges the charge receiving material, which scorotron is disclosed in Japanese Unexamined Patent Publication No. 11946/1994 (Tokukaihei 6-11946) (published on Jan. 21, 1994) (Patent Document 1).
This charging device employing the corona discharge system is used in a pre-transfer charging device for charging a toner image that has not been transferred yet to a transfer medium such as an intermediate transfer body, the recording paper, or the like. Examples of such a charging device are disclosed in Japanese Unexamined Patent Publication No. 274892/1998 (Tokukaihei 10-274892) (published on Oct. 13, 1998) (Patent Document 2) and Japanese Unexamined Patent Publication No. 69860/2004 (Tokukai 2004-69860) (published on Mar. 4, 2004) (Patent Document 3). According to techniques as disclosed in Patent Documents 2 and 3, even if a charge amount is not uniform in the toner image formed on an image bearing member, the charge amount of the toner image is uniformized before the toner image is transferred. Therefore, it becomes possible to suppress a decrease in a transfer margin at the time of transferring a toner image, and also to stably transfer the toner image to a transfer medium.
However, the conventional charging device described above has a plurality of problems. First, the charging device requires not only the discharge electrode but also the shield case, the grid electrode, and the like. Further, it is necessary to ensure a constant distance (10 mm) between the discharge electrode and the charge receiving material. As a result, a large space becomes necessary for providing the charging device. Generally, a developing device, a first transfer device, and the like are provided around a first transfer section, and the photoreceptor, a second transfer device, and the like are provided in front of a second transfer section. Accordingly, a space for the pre-transfer charging device is small. Therefore, in the conventional charging device employing the corona discharge system, it is difficult to make a layout.
Secondly, the conventional charging device employing the corona discharge system generates a large amount of discharge products such as ozone (O3) and nitrogen oxide (NOx). Generation of a large amount of ozone causes (i) ozone smell, (ii) a harmful influence on a human body, (iii) deterioration of members due to strong oxidation power, and the like. Further, when nitrogen oxide is generated, nitrogen oxide as ammonium salt (ammonium nitrate) adheres to the photoreceptor. This causes a defect in an image. Especially an organic photoreceptor (OPC) that is commonly used tends to cause a defect in an image, for example, a white spot or an image deletion because of ozone, NOx or the like.
In view of uniformity of a charge amount of a toner image that has not been transferred yet, a color image forming apparatus, which employs an intermediate transfer system and includes a plurality of transfer sections, preferably has an arrangement in which pre-transfer charging device is provided upstream with respect to each of the transfer sections (a plurality of the first transfer sections, and a second transfer section). However, this is practically difficult in consideration of generation amounts of ozone and NOx.
Furthermore, for the purpose of eliminating ozone, in recent years, a charging device employing a contact electrification system has been used as a charging device for charging the photoreceptor itself. In the contact electrification system, a conductive roller or a conductive brush carries out contact electrification. However, when employing the contact electrification system, it is difficult to carry out charging without damaging the toner image. Accordingly, the corona discharge system which is a non-contact system is used for the pre-transfer charging device. However, in a case where the pre-transfer charging device employing the conventional corona discharge system is provided to the image forming apparatus using the contact electrification system, a characteristic of being ozone free cannot be achieved.
As a technique for reducing a generation amount of ozone, for example, Japanese Unexamined Patent Publication No. 160711/1996 (Tokukaihei 8-160711) (published on Jun. 21, 1996) (Patent Document 4) discloses a charging device including: a large number of discharge electrodes arranged at a substantially equal pitch in a predetermined axial direction; a high voltage power source for applying, to the discharge electrodes, a voltage equal to or higher than a voltage for starting discharge; a resistor provided between an output electrode of the high voltage power source and the discharge electrodes; a grid electrode provided in the vicinity of the discharge electrodes and between the discharge electrodes and the charge receiving material; and a grid power source for applying a grid voltage to the grid electrode. This charging device reduces a generation amount of ozone, by having an arrangement in which a gap between the discharge electrodes and the grid electrode is set to be equal to or less than 4 mm so as to reduce a discharge current.
According to the technique disclosed in Patent Document 4, a generation amount of ozone can be reduced by reduction in the discharge current. However, because the reduction of generation of ozone is not sufficient, approximately 1.0 ppm of ozone is still generated. Further, there is another problem such that discharge may become unstable due to adherence of discharge products, toner, paper powder, or the like to the discharge electrode, or abrasion/deterioration of a tip of the discharge electrode due to discharge energy. Furthermore, a shape of the discharge electrode makes it difficult to clean off the discharge products, the toner, or the paper powder from the discharge electrode.
Moreover, a narrow gap between the discharge electrode and the charge receiving material easily causes non-uniformity of electrification in a longitudinal direction (a direction of the pitch of the discharge electrodes) due to the pitch of the plurality of the discharge electrodes. Here, a shorter pitch of the discharge electrodes may improve the non-uniformity of electrification. However, this increases the number of the discharge electrodes, thereby increasing production cost.
In view of the problems of the conventional charging device, for example, Japanese Unexamined Patent Publication No. 249327/2003 (Tokukai 2003-249327) (published on Sep. 5, 2003) (Patent Document 5) discloses a charging device including an ion generating element (a creeping discharge element) which is provided with a discharge electrode having pointed protrusions on a periphery of the discharge electrode and an inductive electrode in a manner such that the discharge electrode and the inductive electrode sandwich a dielectric body, and which generates ions according to application of a high alternating voltage across the discharge electrode and the inductive electrode (hereinafter, this charging system is referred to as a creeping discharge system). The charging device employing this creeping discharge system is small in size, because the charging device does not have a shield case, a grid electrode, and the like. Further, cleaning of the charging device is easy because a discharging surface of the charging device is flat. Therefore, the charging device also has an advantage in easiness of maintenance.
Here, discharge characteristics of the ion generating element (the creeping discharge element) tend to decline under a high humidity condition. In view of this problem, for example, Japanese Unexamined Patent Publication No. 157447/2004 (Tokukai 2004-157447) (published on Jun. 3, 2004) (Patent Document 6) and Japanese Unexamined Patent Publication No. 237368/2002 (Tokukai 2002-237368) (published on Aug. 23, 2002) (Patent Document 7) disclose a technique for improving the discharging characteristics by providing the ion generating element with a heater member and heating the element to remove absorption moisture. Especially, Patent Document 7 discloses a technique in which an inductive electrode is electrified to generate Joule heat, thereby doubling as a heater. With this technique disclosed in Patent Document 7, it becomes possible to make the ion generating element more compact and to reduce costs than with the technique of providing an additional heater element independently.